Novak Conversions Jeep Wrangler TJ radiator

How to recalibrate your speedometer after changing tire sizes or gears

AWTJ2000 said:
Reading faster on the GPS than the speedo after I replaced the part.
Click to expand...

So going 65 on the dash, 70 on the GPS?

If that’s the case, you need a 29 tooth gear.

65 * 31 / 70 = 28.8

I never recommend anybody buy their gear based on a chart or nominal tire size. Math is always the best way to get it accurate. Too many incorrect charts out there and too many fluctuations in real world tire diameters.
 
machoheadgames said:
So going 65 on the dash, 70 on the GPS?

If that’s the case, you need a 29 tooth gear.

65 * 31 / 70 = 28.8

I never recommend anybody buy their gear based on a chart or nominal tire size. Math is always the best way to get it accurate. Too many incorrect charts out there and too many fluctuations in real world tire diameters.
Click to expand...

Thanks so much for the response!

We actually had it flipped- 65 on speedometer, and 62 on the GPS.
 
machoheadgames said:
So going 65 on the dash, 70 on the GPS?

If that’s the case, you need a 29 tooth gear.

65 * 3 / 70 = 28.8

I never recommend anybody buy their gear based on a chart or nominal tire size. Math is always the best way to get it accurate. Too many incorrect charts out there and too many fluctuations in real world tire diameters.
Click to expand...
 
Last edited:
So I replaced the speedometer gear with a 31 tooth gear, per the graph (3.73 gears and 33x12.50/15 tires). Originally my speedometer was 10% slow against my GPS Speed App. Now, however, I am 3 mph fast at all speeds, 20, 40, 50??? This doesn’t make any sense? Any suggestions?
 
I replaced my speedo gear with a 35 tooth, and it runs fairly close to the GPS UNTIL i get up to 60+ MPH, then it’s like 5 MPH low now on the speedometer.
 
I'm confused. I have 99, 4.0, 3 speed auto and 33s 4:10 gears. What speedo gear do I need?
 
machoheadgames said:
Usually 35 or 36. What tooth do you have now and how much are you off by at a set speed?
Click to expand...

Thanks. I'll have to check it's original for 3:07 gears and 32rh. I'm off like 15-20 mph. Been like that for a couple years since my regear. Really should get around to correcting it.
 
My TJ had the original 36 tooth gear (3.73). 50MPH on the speedo was 57MPH on my GPS unit.

50/57*36 = 31.57 = 32

I installed a 32 tooth gear and now 50MPH on the speedo is 51MPH on my GPS unit. (y)

Andy
 
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Harold&Cat said:
This is a valuable resource . I'll be installing the 43 tooth gear today after regearing to 4.88 and 33 inch tires . We'll see how it goes :)
Click to expand...

Why 43 if chart says 41 for 4.88 and 33 inch tires?
(I am looking to do the same thing after regear)
Did you do math instead? :LOL:
 
Well one thing I just found was that the table on here for 4.56 and 33’s is different tooth count than the one on quadratech. One is 39 vs 34, which is about 10 mph at 50
 
Gee, I always thought I was doing the speed limit and everyone who passed me was a speeding bunghole. Little did I know I was actually doing 12% less than what my speedo was showing. Well, at least I never got a speeding ticket. Everyone behind me was probably muttering "Gosh darn slow Jeep!" 😁
 
Sorry for the late reply . Yes , it helped greatly . The speedometer is very close . If I remember correctly , it's within one or two miles per hour . I'll check it next time I'm out and post it .
 
Novak Conversions Jeep Wrangler TJ radiator
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